The present invention relates to an apparatus and method for diagnosing occurrence of failure in a variable valve timing control system for an internal combustion engine in which the variable valve timing control system is mounted.
U.S. Pat. Nos. 5,203,289 (issued on Apr. 20, 1993), 5,150,671 (issued on Sep. 29, 1992), 5,209,193 (issued on May 11, 1993), 5,205,248 (issued on Apr. 27, 1993), 5,195,471 (issued on Mar. 23, 1993), 5,228,417 (issued on Jul. 20, 1993), 5,228,417 (issued on Jul. 20, 1993), 5,247,914 (issued on Sep. 28, 1993), 5,197,421 (issued on Mar. 30, 1993), 5,263,442 (issued on Nov.23, 1993), 5,203,290 (issued on Apr. 20, 1993), 5,309,873 (issued on May 10, 1994), and 5,353,755 (issued on Oct. 11, 1994) exemplify variable valve timing control systems for internal combustion engines.
In general, in an internal combustion engine, if a timing at which an intake valve(s) is closed is earlier than usual, an intake air sucking efficiency is improved in low engine speed or middle speed ranges. On the other hand, in a high speed range, if the timing at which the intake valve(s) is closed is later than usual, the intake air sucking efficiency is improved. Consequently, an improvement in an engine output torque can be achieved.
Therefore, such a variable valve timing control system as described in each of the above-identified United States Patents has been proposed in which a variable valve timing mechanism is operated according to an engine driving condition, the variable valve timing mechanism switching an open-and-closure timing of an engine intake valve(s). When the engine falls in a high engine revolution speed range and in a high load condition, an overlap time duration during which the intake valve and exhaust valve are open is elongated so that an air filling efficiency can be improved utilizing an intake air inertia force.
Such a variable valve timing control system as described above is abbreviated VTC and is called an NVCS in a Japanese Service Manual (New Model Introduction of Nissan "Cima" (FGY32-1) published on August, 1991.
In the VTC, for example, a control signal is output from a control unit, the control unit receiving an engine revolution speed indicative signal and intake air quantity indicative signal, to a variable timing control solenoid. The solenoid is excited (in an ON state) or turned off (in an OFF state) so that a hydraulic is controlled to operate the variable valve timing mechanism, thus carrying out a switching control over the open-and-closure timings of the intake valve(s).
In the VTC described above, in a case where an electrical failure in an output system of the control signal and/or a mechanical failure in the variable valve timing mechanism occurs so that a malfunction of the variable valve timing control system results, a higher engine revolution is required in order to achieve the same engine output as when the VTC is operated. In addition, an exhaust gas emission quality is worsened.
Hence, it is essential to carry out a self diagnosis for the VTC with less cost.